The photochemistry of anthracene and 9-methylanthracene in the room-temperature molten salts

The photochemistry of anthracene and 9-methylanthracene has been examined in the room-temperature molten salts: aluminum chloride/1-ethyl-3-methyl-1H-imidazolium chloride (EMIC) and aluminum chloride/N-1-butylpyridinium chloride (BPC). In the deoxygenated basic melts (55 mol % EMIC or BPC), anthracene reacts photochemically to form the normal (4+4) dimer, exclusively. In the deoxygenated acidic melt (45 mol % EMIC) the reaction takes a different course, yielding a variety of monomeric and dimeric, oxidized, neutral and reduced products. This unusual photoreaction is initiated by electron transfer from the excited state of anthracene to protonated anthracene, which is generated in low concentration by the reaction of anthracene with trace amounts of Al&sub2;Cl&sub7;HCl&sup-;, a powerful Bronsted acid in the acidic melt. Photolysis of the more easily oxidized 9-methylanthracene in the deoxygenated basic melts also occurs in part by photoinduced electron transfer. The imidazolium ion (EMI&sup+;) and the pyridinium ion (BP&sup+;) proved to be the electron acceptors in this instance.